OAuth2 password flow in FastAPI - Mini Project: Build & Apply

OAuth2 Password Flow with FastAPI

📖 Scenario: You are building a simple API that requires users to log in using their username and password. You want to secure your API endpoints using OAuth2 password flow, which is a common way to handle user authentication.

🎯 Goal: Create a FastAPI app that implements OAuth2 password flow. You will set up user data, configure OAuth2 password bearer, write the login logic to verify users, and protect an API route that only logged-in users can access.

📋 What You'll Learn

Create a dictionary called fake_users_db with one user entry: username alice and password secret123

Create an OAuth2PasswordBearer instance called oauth2_scheme with token URL /token

Write a function authenticate_user that takes username and password and returns True if they match the user in fake_users_db, else False

Create a /token POST route that accepts form data username and password, uses authenticate_user, and returns a JSON with access_token and token_type

Create a protected /users/me GET route that requires a valid token from oauth2_scheme and returns the current username

💡 Why This Matters

🌍 Real World

OAuth2 password flow is commonly used in APIs to securely authenticate users with username and password, issuing tokens for session management.

💼 Career

Understanding OAuth2 password flow is essential for backend developers building secure APIs and services that require user authentication.

1

Set up user data dictionary

Create a dictionary called fake_users_db with one user entry: key alice and value another dictionary with key password and value secret123.

FastAPI

# Create the fake_users_db dictionary with user alice and password secret123
# Your code here

Hint

Use a dictionary with username as key and a nested dictionary with password key.

2

Configure OAuth2PasswordBearer

Import OAuth2PasswordBearer from fastapi.security and create an instance called oauth2_scheme with tokenUrl="/token".

FastAPI

fake_users_db = {
    "alice": {"password": "secret123"}
}

# Import OAuth2PasswordBearer and create oauth2_scheme with tokenUrl='/token'
# Your code here

Hint

Use OAuth2PasswordBearer with the token URL where users will send their login data.

3

Write user authentication function

Define a function called authenticate_user that takes username and password. It returns True if username is in fake_users_db and the password matches, otherwise returns False.

FastAPI

from fastapi.security import OAuth2PasswordBearer

fake_users_db = {
    "alice": {"password": "secret123"}
}

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")

# Define authenticate_user function
# Your code here

Hint

Check if username exists and password matches, then return True, else False.

4

Create token and protected routes

Import FastAPI, Depends, and HTTPException from fastapi. Create a FastAPI app instance called app. Add a POST route /token that accepts form data username and password using OAuth2PasswordRequestForm. Use authenticate_user to verify credentials. If valid, return JSON with access_token set to username and token_type set to bearer. If invalid, raise HTTPException with status 401. Then add a GET route /users/me that depends on oauth2_scheme to get the token and returns JSON with username equal to the token.

FastAPI

from fastapi.security import OAuth2PasswordBearer

def authenticate_user(username: str, password: str) -> bool:
    if username in fake_users_db and fake_users_db[username]["password"] == password:
        return True
    return False

fake_users_db = {
    "alice": {"password": "secret123"}
}

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")

# Create FastAPI app and routes
# Your code here

Hint

Use OAuth2PasswordRequestForm to get form data, verify user, raise 401 if invalid, else return token. Protect /users/me with oauth2_scheme.

Practice

1. What is the main purpose of the OAuth2 password flow in FastAPI?

easy

A. To allow users to log in by sending their username and password directly to the app.

B. To register new users automatically without credentials.

C. To refresh access tokens without user interaction.

D. To encrypt user passwords before storing them.

Solution

1. Step 1: Understand OAuth2 password flow purpose

   This flow lets users send their username and password to the app to get an access token.

2. Step 2: Compare options with flow purpose

   Only To allow users to log in by sending their username and password directly to the app. describes this direct login method; others describe different features.

3. Final Answer:

   To allow users to log in by sending their username and password directly to the app. -> Option A

4. Quick Check:

   OAuth2 password flow = direct login [OK]

Hint: Password flow means user sends username and password [OK]

Common Mistakes:

• Confusing password flow with token refresh
• Thinking it registers users automatically
• Assuming it encrypts passwords by itself

2. Which FastAPI import is used to handle OAuth2 password flow form data?

easy

A. from fastapi.security import OAuth2PasswordBearer

B. from fastapi.security import OAuth2PasswordRequestForm

C. from fastapi.security import HTTPBasicCredentials

D. from fastapi.security import APIKeyHeader

Solution

1. Step 1: Identify form class for password flow

   FastAPI uses OAuth2PasswordRequestForm to parse username and password from form data.

2. Step 2: Check other imports

   OAuth2PasswordBearer is for token extraction, HTTPBasicCredentials is for basic auth, APIKeyHeader is for API keys.

3. Final Answer:

   from fastapi.security import OAuth2PasswordRequestForm -> Option B

4. Quick Check:

   Form data handler = OAuth2PasswordRequestForm [OK]

Hint: Password flow form uses OAuth2PasswordRequestForm [OK]

Common Mistakes:

• Using OAuth2PasswordBearer instead of RequestForm
• Confusing HTTPBasicCredentials with OAuth2 forms
• Importing unrelated security classes

3. Given this FastAPI endpoint using OAuth2 password flow, what will be the response if username is 'alice' and password is 'secret'?

from fastapi import FastAPI, Depends
from fastapi.security import OAuth2PasswordRequestForm

app = FastAPI()

@app.post('/token')
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    if form_data.username == 'alice' and form_data.password == 'secret':
        return {'access_token': 'token123', 'token_type': 'bearer'}
    return {'error': 'Invalid credentials'}

medium

A. {'access_token': 'token123', 'token_type': 'bearer'}

B. {'error': 'Invalid credentials'}

C. HTTP 422 Unprocessable Entity error

D. Empty response with status 204

Solution

1. Step 1: Check input credentials against condition

   The code checks if username is 'alice' and password is 'secret'. Given inputs match this.

2. Step 2: Determine returned response

   Since condition is true, it returns the access token dictionary with 'token123' and 'bearer'.

3. Final Answer:

   {'access_token': 'token123', 'token_type': 'bearer'} -> Option A

4. Quick Check:

   Correct credentials = access token response [OK]

Hint: Match username and password to get token response [OK]

Common Mistakes:

• Assuming error response for correct credentials
• Confusing HTTP errors with normal returns
• Ignoring the if condition logic

4. What is wrong with this FastAPI OAuth2 password flow code snippet?

from fastapi import FastAPI, Depends
from fastapi.security import OAuth2PasswordRequestForm

app = FastAPI()

@app.post('/token')
async def login(form_data: OAuth2PasswordRequestForm):
    if form_data.username == 'bob' and form_data.password == 'pass':
        return {'access_token': 'abc', 'token_type': 'bearer'}
    return {'error': 'Invalid'}

medium

A. Endpoint should use GET method instead of POST

B. Incorrect import of OAuth2PasswordRequestForm

C. Return type should be a string, not dict

D. Missing Depends() in function parameter for form_data

Solution

1. Step 1: Check function parameter for dependency injection

   OAuth2PasswordRequestForm must be wrapped with Depends() to extract form data properly.

2. Step 2: Verify other parts

   Imports are correct, return type as dict is valid JSON response, POST method is correct for token requests.

3. Final Answer:

   Missing Depends() in function parameter for form_data -> Option D

4. Quick Check:

   Use Depends() to get form data [OK]

Hint: Always wrap OAuth2PasswordRequestForm with Depends() [OK]

Common Mistakes:

• Forgetting Depends() causes runtime errors
• Using GET instead of POST for token endpoint
• Thinking return must be string, not dict

5. You want to secure a FastAPI endpoint so only users with a valid OAuth2 password flow token can access it. Which approach correctly uses OAuth2PasswordBearer and token verification?

from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer

app = FastAPI()
oauth2_scheme = OAuth2PasswordBearer(tokenUrl='token')

def verify_token(token: str):
    if token != 'validtoken':
        raise HTTPException(status_code=401, detail='Invalid token')

@app.get('/secure-data')
async def secure_data(token: str = Depends(oauth2_scheme)):
    verify_token(token)
    return {'data': 'secret info'}

hard

A. Incorrect: verify_token should return True/False, not raise exceptions.

B. Incorrect: tokenUrl should be '/secure-data' not 'token'.

C. Correct: uses OAuth2PasswordBearer and verifies token before returning data.

D. Incorrect: OAuth2PasswordBearer cannot be used with GET endpoints.

Solution

1. Step 1: Check OAuth2PasswordBearer usage

   oauth2_scheme is created with tokenUrl='token', which is correct for password flow token endpoint.

2. Step 2: Verify token validation logic

   verify_token raises HTTPException on invalid token, which is proper for access control.

3. Step 3: Confirm endpoint dependency and response

   secure_data depends on oauth2_scheme to get token, verifies it, then returns protected data.

4. Final Answer:

   Correct: uses OAuth2PasswordBearer and verifies token before returning data. -> Option C

5. Quick Check:

   Use OAuth2PasswordBearer + verify token = secure endpoint [OK]

Hint: Use OAuth2PasswordBearer with tokenUrl and verify token [OK]

Common Mistakes:

• Setting wrong tokenUrl in OAuth2PasswordBearer
• Not raising exceptions on invalid token
• Thinking OAuth2PasswordBearer can't be used with GET